Chemical bonding in oblatonido ditantalaboranes and related compounds

Abstract

The recently discovered ditantalaboranes Cp₂Ta₂B_nH_n+6 (n = 4, 5) are isoelectronic with the previously discovered dimetallaboranes Cp₂M₂B_nH_n+4 of the group 6 metals Cr, Mo, and W where Cp = η⁵-cyclopentadienyl or substituted cyclopentadienyl. Their oblatonido polyhedral structures can be derived from the oblate (flattened) deltahedra of the oblatocloso dirhenaboranes Cp₂Re₂B_n+1H_n+1 by removal of an equatorial BH vertex with adjustment of the skeletal electron count by changing the metal atoms and adding hydrogen atoms. In these oblatocloso dirhenaborane deltahedra, the approximately antipodal rhenium atoms are close enough together to form a formal Re=Re double bond with lengths in the range 2.69–2.82 Å. Similarly, short M=M distances are maintained in the related oblatonido derivatives Cp₂Ta₂B_nH_n+6 (n = 4, 5) and Cp₂M₂B_nH_n+4 (M=Cr, Mo, W). However, the synthesis of Cp₂Ta₂B_nH_n+6 (n = 4, 5) from CpTaCl₄ + LiBH₄/BH₃ also gives a less-reduced product Cp₂Ta₂Cl₂B₅H₁₁ with a longer Ta–Ta distance of ~3.2 Å. This may be regarded as a formal single bond bridged by one of the hydrogen atoms. Vertices of degree 5 (excluding terminal atoms/groups but not edge-bridging hydrogens) are sites of highest stability/lowest chemical reactivity not only in metal-free boranes but also in the dimetallaboranes discussed in this paper. For example, all four boron vertices in Cp₂Ta₂B₄H₁₀ have the favorable degree or 5.